170 SCIENCE PROGRESS 



progress in the study of heredity during the last twenty years : the names of 

 Mendel and de Vries are inseparably connected with this method ; (3) the method 

 of cytology, the value of which the authors fully appreciate ; and (4) the method 

 of experimental morphology. In genetics this last method has for its object the in- 

 vestigation of the development of the individual as it is related to pro blems of 

 variation and heredity. Work along these lines has as yet scarcely commenced. 



The greater part of Part I naturally deals with Mendelism, and special attention 

 is directed to recent developments in Mendelian analysis ; naturally much 

 reference is made to the work of Morgan on Drosophila, which has extended 

 knowledge in the subject so vastly. Apart from this the authors cite a large amount 

 of work on different species and by different workers, and have exercised great 

 wisdom in their choice of subject-matter. Other matters discussed in the part 

 on Fundamentals are variation in general, the statistical study of variation, 

 in which the principles of biometry are described, species hybridisation involving 

 questions of hybrid form and vigour and sterility, pure lines and mutations. 



These Fundamentals occupy nearly half the book. In the second part, on 

 Plant Breeding, the questions of varieties and their origin, selection, hybridisa- 

 tion and hybrids, and mutations in plant breeding are among practical problems 

 dealt with. A chapter is also devoted to the composition of plant populations 

 in which methods of reproduction and fertilisation are considered. Two interesting 

 chapters are devoted to graft hybrids and bud selection, while a chapter on the 

 breeding of disease resistant plants is very stimulating. 



The scope of Part III, which deals with Animal Breeding, is similar to that 

 of Part II, selection, hybridisation, disease, the inheritance of acquired characters 

 as well as breeding methods being among the questions discussed. In regard to 

 the inheritance of acquired characters, the authors, after a careful sifting of the 

 evidence, come to the orthodox conclusion that there is no experimental de- 

 monstration of the inheritance of any acquired character. The questions of sex 

 and fertility in animals are very adequately dealt with. 



As an example of how the different methods of genetics may be correlated 

 in the solution of problems may be mentioned the case of the sterility of the mule, 

 a well-known result of experimental breeding. It appears that the germ-cells 

 of the horse have 19 chromosomes, and those of the ass 32 or 33. Consequently 

 " reduction divisions in the mule are prevailingly abnormal as to chromatin 

 distribution, and no functional spermatozoa appear to be produced." 



The book is clearly written, but in the chapter on variation there appears to 

 be some confusion between an external factor or condition and a stimulus, which 

 is a change in the condition. Also the division of variations into four classes, 

 morphological, physiological, psychological, and ecological, does not seem very 

 happy— as, for instance, variations in leaf colour due to habitat might be classed 

 either as morphological, physiological or ecological, at any rate in some cases. 



It is impossible in a small space to indicate adequately any more than the 

 general scope of this book, but enough has been said to make it clear that the 

 work supplies a real need. The whole ground of genetics is covered, a wide range 

 of material has been chosen for subject-matter, as is indicated sufficiently in the 

 length of literature cited, and a good balance is maintained in regard to the space 

 devoted to different questions. The work will be invaluable not only to students 

 and teachers of biology and agriculture, but also to research workers both 

 in genetics and plant and animal breeding, and in plant and animal physiology. 

 In preparing a text-book on a subject which has undergone such enormous 

 and rapid development, the authors imposed on themselves a great and difficult 



